It is common to protect LEDs from electrostatic discharge (ESD) or other high voltage transient signals by connecting back-to-back zener diodes in parallel with the LED. If a reverse voltage across the LED is above the zener breakdown voltage, the current is shunted through the zener diodes to the power supply and the LED is protected. Such a protection circuit is referred to as a transient voltage suppressor (TVS).
It is common to interconnect LED dice in series so that each LED drops a forward voltage and the LEDs operate at the same current. It is more efficient to generate a high voltage and low current than a high current and low voltage. Such a series connection is common in high brightness applications such as illumination and backlighting. Many LED dice may be connected in series so as to be directly connected to a 120v AC mains voltage.
Prior art FIG. 1 illustrates each LED die in a series connection being protected by an identical set of back-to back zener diodes. In FIG. 1, the LED dice are represented by diodes D1-Dn, and the zener diodes are represented by Q1-Q2n. The string of LEDs is turned on by applying a voltage across the pins 1 and 2 greater than the sum of the forward voltage drops of the LEDs. A maximum forward voltage drop for turning on an InGaN LED is about 4-5 volts. Since each set of zener diodes is identical, each zener diode must have a breakdown voltage above the maximum combined forward voltages of all the LEDs so as not to break down under normal operating conditions.
It is known to form the zener diodes in a silicon substrate 12 (also known as a submount), on which is mounted a plurality of LED dice connected in series. The substrate 12 has a dielectric layer (e.g., oxide) over its top surface and a top metal pattern over the dielectric layer that interconnects the LED electrodes to form a series interconnection. The metal pattern also connects the zener diodes to the LED electrodes. The metal pattern provides leads or pads on the silicon substrate for connection to a power supply or for connection to another substrate having additional series-connected LEDs.
FIG. 2 illustrates the formation of back-to-back zener diodes (e.g., Q1 and Q2 in FIG. 1) in parallel with an LED (e.g., D1). The zener diodes Q1 and Q2 are typically formed by ion implanted n+ regions 16 and 18 in a p+ silicon substrate 12. The ion implantation doping level is identical for all the zener diodes in FIG. 1, and the zener diodes have the same breakdown voltage. The distance d between the regions 16 and 18 (d is identical for all the zener diodes pairs) must be large enough so that the snapback phenomenon does not occur prior to the zener diode breakdown. The snapback phenomenon is a form of breakdown between regions 16 and 18. In snapback, the parasitic NPN transistor formed by the n+ region 16, the p+ substrate 12, and the n+ region 18 turns on when enough carriers are injected into the p+ substrate base due to an ESD event or an overvoltage. When the NPN transistor turns on, a current flows between the regions 16 and 18, resulting in more carriers being injected into the base. This creates a positive feedback, and the NPN transistor latches on, causing even more carriers to flow. This forms a shunt path in parallel with the LED, which wastes power and affects overall LED performance. By increasing the distance between the n+ regions, the gain of the NPN transistor is greatly reduced due to limited carrier lifetime, which prevents the positive feedback from occurring, thus preventing snapback.
The width W of the regions 16 and 18 directly affects the series resistance through the zener diode pair. It is desirable that the resistance be low such that the zener diodes quickly conduct a high current as soon as the voltage exceeds the breakdown voltage. A high value series resistance (W is small) limits the current through the zener diodes so the LED dice have less protection against high voltage transient signals.
The available silicon substrate area for forming two zener diodes per LED die is limited, especially for a multi-junction LED die having a small footprint (e.g., 1 mm2). Each set of zener diodes is typically formed either under or next to the LED die it protects. When more and more LED junctions are connected in series, the supply voltage must increase. As the operating voltage increases, the substrate p doping must decrease to achieve the required increase in zener breakdown voltage. This requires a larger minimum spacing d between the zener diodes to avoid snapback from occurring before the zener diode pair breaks down, since it takes less charge to form a current path through the substrate between the zener diode regions. Therefore, when LED dice are connected in series on a silicon substrate within a small footprint (e.g., 1 mm2), the silicon surface area underneath the dice for forming the zener diodes may be inadequate according to design rules in principle for good transient voltage protection of the LEDs.
After the silicon substrate (a wafer) is processed to create the zener diodes and the metallization pattern, LED dice are mounted on the substrate, such as by using ultrasonic bonding to bond the LED electrodes to the substrate pads. The LEDs are typically flip-chips with both electrodes formed on the bottom, and light is emitted from the top surface. The growth substrate (e.g., sapphire) is then removed from the top surface of the LEDs, such as by laser lift-off or other well known techniques. This exposes the top n-layer of the LEDs.
It is known to precision-roughen the exposed n-layer to increase light extraction (reduces internal reflection). One way to etch the LED surface to roughen it is to perform photo-electrochemical etching (PEC etching). PEC etching is well known for GaN LEDs. In one type of PEC etching process, the top surface of the LED is electrically biased, and the LED is placed in an electrolyte solution (e.g., KOH) containing a biased electrode. The LED is then exposed to ultraviolet light. The UV light creates electron-hole pairs in the GaN, and the holes migrate to the surface by diffusion and under the influence of the electric field. The holes react with the GaN and the electrolyte at the surface to break the bonds of the GaN, resulting in controlled roughening of the surface. The etching also removes damaged GaN that is created near the growth substrate/n-layer interface.
Since the p+ silicon substrate is electrically connected to the exposed n-layer of the LEDs (e.g, D1 in FIG. 1) when the zener diodes (e.g., Q2 in FIG. 1) connected to the n-electrodes are forward biased, the n-layer may be biased during the PEC etch by connecting a positive voltage to the p+ substrate, via a bottom metal electrode. A small current then flows from the substrate, through the zener diode, through the n-layer, through the electrolyte, and through the electrolyte electrode to perform the PEC etching.
After the PEC etching, lenses, phosphor, or other optical elements may be formed over the LEDs on a wafer scale. The silicon wafer is then diced to separate out the individual substrates, each substrate containing a plurality of LED junctions connected in series and each LED junction being protected by a set of zener diodes.
What is needed is a technique to form more robust zener diodes in the silicon substrate for improved transient voltage suppression yet still enable the top semiconductor layer of the LEDs to be etched by PEC etching.